The operation of rotating electrical machines, such as motors and generators, is based on magnetic interaction between a stator and a rotor of the machine. Generally magnetization of the poles of the stator and the rotor can be carried out by using permanent magnet pieces or by using windings carrying current. In general, in the case of synchronous machines, the stator comprises the windings and the rotor poles are magnetized either by using permanent magnets or current carrying windings. When a synchronous machine is used as a generator, the rotor is rotated by mechanical power and a rotating magnetic field induces voltage to stator windings. When the synchronous machine is used as a motor, an alternating current produces a rotating magnetic field into the stator and the magnetized rotor follows the movement of the field and thereby rotates with the rotating magnetic field.
The use of electrical magnetization or field coils in the rotor has the advantage that the magnetization can be controlled by controlling the current fed to the rotor poles. This current is referred to as a field magnetizing current, excitation current or rotor current. The purpose of this current is to produce the required magnetic field to the rotating rotor.
Field magnetization can be produced using either brushless magnetization or magnetization with brushes. In the magnetization with brushes, the current is fed from a stationary power source to the rotating rotor by using brushes that carry the current from a stationary side to the rotating rotor.
FIG. 1 shows a basic structure used in brushless magnetization. In the brushless magnetization, a magnetizing machine or an exciter is used for producing a magnetizing current IR to a field winding 3 of the main machine. A rotor winding 1 of the magnetizing machine rotates together with the rotor of the main machine while a stator winding 2 of the exciter is energized by an excitation current Im. A current generated to the rotating winding is rectified with a diode bridge, and the rectified current, i.e. the rotor current IR, is further fed to the field winding 3. The rotor current IR is thus controlled indirectly by controlling the excitation current Im.
FIG. 1 also shows the rotating parts as a dotted box 5 and a device 6 used for controlling the magnetizing current. This device is generally referred to as an AVR (automatic voltage regulator). AVR receives multiple inputs (not shown) and controls the magnetization of the machine according to the inputted signals. FIG. 1 further shows stator windings 7 of the main machine from which a three phase U, V, W voltage is obtainable when the machine in question is a generator.
With the above brushless excitation, the structure of the machine can be quite complicated owing to the controlled magnetizing current. The AVR contains a power stage that is capable of producing currents in the range of tens of amperes. The power stage has a certain amount of losses and the whole AVR system is large. Further, the AVR system involves a separate power supply, e.g. transformers, a pilot exciter and so on, increasing the complexity and space required by the system.